The present invention relates to a method for producing a package-type semiconductor assembly, and more particularly relates to an improvement in the production of a package casing for a semiconductor assembly in which one or more semiconductor elements are built in a closed package in electric connection to outside devices via conductive leads extending through the walls of the package.
In one typical example of a semiconductor assembly equipped with IC chips, LSI chips, photoelectric elements or crystal oscillators, islands and inner leads mounted on a ceramic base are overlaid with a ceramic ring and low melting point glass is used for bonding the ceramic ring to the parts on the ceramic base.
In the case of such a conventional semiconductor assembly, the price of the product is much swayed by use of expensive ceramics for the base and the ring. Since the ceramic parts are prepared by cutting and polishing, the parts are in most cases inevitably accompanied with much powdery dust. Presence of such powdery dust often wields malign influence on the electric characteristics of the device, thereby lowering the functional reliability of the product.
In an attempt to remove such disadvantages resulted from use of ceramics, a U.S. Pat. No. 3,340,347 to Spiegler proposed production of a closed package-type semiconductor assembly. In the process of this earlier patent, a metallic coating of an electroconductive material is applied to on surface of a flat dielectric substrate, predetermined portions of the metallic coating are removed to form contact plates and conductive leads are disposed in contact with the contact plates in a manner so that they extend beyond the edges of the substrate. A glass ring is disposed about the periphery of the substrate in contact with the conductive leads and extends over the bond between the conductive leads and the contact plates. The glass ring is then fused to the substrate to form an enclosure body.
In fact, this proposed method well removed the disadvantages resulted from use of ceramics. Despite such advantages, however, the method is accompanied with other disadvantages. For fusion of the glass ring to the substrate, temperature must be controlled carefully so as to melt the glass only without any influence on the substrate. Moulds need to be used to keep the shape of the glass ring during the fusion process. Further, since the conductive leads cannot be bonded directly to the substrate, a metallic coating must be additionally formed on the surface of the substrate. Production cost is raised due to the multi-staged production process and delicate temperature control.